This invention relates generally to systems and methods for restricting movement in flexible ducts, and, more generally, to methods and systems for increasing the natural frequency of a flexible duct.
Exhaust systems for rotorcraft or other vehicles may require features that mix hot exhaust air and cooler ambient air to limit temperatures of the surrounding vehicle structure or to limit the heat signature of the vehicle. One method of achieving cooler exhaust temperatures is to attach a flow mixer to the outlet of the exhaust system. At least some known flow mixers include a plurality of lobes, or “flutes” that promote mixing of the two air streams. A drawback of a lobed or fluted design is that the stiffness of the flow mixer is necessarily reduced. At engine or rotor frequencies that align with the natural frequency of the flow mixer, the flow mixer is subject to high vibrational loads that cause deflections in the flow mixer structure. The alignment of natural frequencies and rotor frequencies (resonance) leads to increased deflection and stresses that severely limit or prohibit a lobed design. Further, static or dynamic pressures on the walls of the lobes may cause large deflections in the lobe areas and negatively impact flow mixing.
Duct size is a significant driver of natural frequency. At least some known exhaust systems use relatively small size flow mixers that potentially avoid critical frequencies without additional stiffening features. However, such small size flow mixers enable less efficient mixing of hot exhaust air and cooler ambient air. Furthermore, at least some known exhaust systems may use multiple small size flow mixers to increase the mixing efficiency. However, multiple flow mixers increases the manufacturing and maintenance costs associated therewith and also increases the weight of the exhaust system.